On the matter of predictability, I remember working on a product called SigmaPlot. In a nutshell, it was a scientific charting product with a worksheet to contain data, and an exceedingly complex "menu" structure of options to design the very esoteric charts needed by researchers and scientists. As a product, it is still going strong, but with a lot of design improvements over the original version.
The problem was that unless you were a constant user of the product, you forgot how to do things, so making charts was an exercise in frustration for the infrequent or "some-time" user. The product was immensely popular for the charting that you could do with it, but our users had a love-hate relationship with it. In fact, what often happened was that a single researcher at a facility became the SigmaPlot Guru, and others would go to him or her for their charts. Good for stoking up the ego of that researcher, but bad for everyone else, including Jandel—the company that made SigmaPlot—since we could only sell one unit to the guru, instead of units to all of the researchers who had charting needs.
The barrier to use was the immense learning curve needed to use the product, and it did require you to drink the kool-aid to use. When I was first hired, I lobbied for the then-novel "Macintosh" (and up-and-coming "Windows operating environment") interface, sensing that this was the way things were headed. It wasn't a hard pitch, as I think almost everybody there was already game to the idea. There were other advantages, too: the only-when-needed popup menus were much more economical on the limited screen real estate than a large "choices 1 through 10" double-spaced menu permanently down the left side of the screen, but the big win was that Macs were seen as far easier to use than their IBM-PC counterparts, and were making in-roads in the still-new personal computer industry. Interface metaphors—like a top-of-screen menu bar, copy and paste, and object-verb commands (select something, then issue the command against it)—were increasingly what many users had come to expect from computers, so these ways of relating to computers carried forward to their use of SigmaPlot as well.
It was a radical change, but one that paid dividends. The previous version of SigmaPlot had a userbase of around eight thousand users... the new version tripled the userbase to almost twent-five thousand in a few short months. Clearly, people found it much easier to use, as our sales figures showed. All because we didn't require customers to drink the kool-aid of yet another arcane command and interface structure. Looking at the product, they could predict with reasonable certainty what effects their actions would have in the product. And that's what usability all about.
That said, SigmaPlot wasn't universally loved. We did get hate mail, mostly from those disenfranchised gurus who were finding their colleagues happily producing very good charts without them, but also from ideologs who proclaimed "if I wanted a Macintosh, I would have bought a Macintosh!"